In recent years, thin and flat display devices have been needed as display devices used for televisions, cellular phones, digital cameras, and the like. As the display devices satisfying this need, display devices using self-light emitting elements have attracted attention. One of the self-light emitting elements is a light emitting element utilizing electroluminescence (EL), and this light emitting element includes a light emitting material interposed between a pair of electrodes and can provide emission from the light emitting material by voltage application.
Such a self-light emitting element has advantages over a liquid crystal display element, such as high visibility of the pixels and no need of backlight, and is considered to be suitable as a flat panel display element. Another major advantage of such a light emitting element is that it can be manufactured to be thin and lightweight. In addition, extremely high response speed is also a feature.
Further, such a self-light emitting element can be formed into a film shape; therefore, plane emission can be easily obtained by forming a large-area element. Since this feature is hard to obtain from a point light source typified by an incandescent lamp or an LED or a linear light source typified by a fluorescent lamp, the self-light emitting element has high utility as a plane light source which is applicable to a lighting system or the like.
Light emitting elements utilizing electroluminescence are classified according to whether a light emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter as an inorganic EL element.
Inorganic EL elements are classified according to their element structures into dispesion-type inorganic EL elements and thin-film inorganic EL elements. They differ in that the former includes a light emitting layer in which particles of a light emitting material are dispersed in a binder, and the latter includes a light emitting layer formed from a thin film of a light emitting material; however, they are share a common feature in that both require electrons accelerated by a high electric field. Note that a mechanism of emission includes a donor-acceptor recombination emission which utilizes a donor level and an acceptor level and a localized emission which utilizes inner-shell electron transition of metal ions. In general, it is often the case that dispersion-type inorganic EL elements employ donor-acceptor recombination emission, and thin-film inorganic EL elements employ localized emission.
Such inorganic EL elements have an advantage of having longer life than organic EL elements. However, they require electrons accelerated by a high electric field for the light emitting layer, so in general it is necessary to apply a voltage of several hundred volts to the light emitting element. For example, a high-luminance blue light emitting inorganic EL element which is necessary for a full-color display has been developed in recent years; however, it requires a drive voltage of 100 V to 200 V (for example, see Reference 1: Japanese Journal of Applied Physics, 1999, Vol. 38, pp. L1291-L1292). Therefore, inorganic EL elements consume a large amount of electric power, so it is difficult to apply them to small and medium-sized displays, for example, to displays of cellular phones or the like.